Cutting a Imperial Thread

[philbur]

Ah nope, it looks to me that compound is set slightly under 29.5 degrees Phil, so I'll mark that photo as a fail. Not so easy maybe.

That set-up took all of five seconds, I guess if I had taken 10 seconds I could have achieve a standard that even you could not fault. So maye still easy enough for most people. It is of course more than accurate enough for the threading procedure as described, splitting of hairs is not a requirement in this particular case.

Phil

[pipeclay]

Just taking a guess here as to why he couldn't use the centre he had.

1) Was using a live centre that's body was too large for him to get down to thread size when using a 4 way tool post.

2) Was using a threading tool that maybe was a little too short to stick out of the tool post further.

3) Didn't consider plunge cutting the thread.

[bwal74]

This bloke had clearance issues, with the compound / top slide and a centre / half center issue. It's bugged me all day.


I am at a lose, what that means. Turning the top / compound slide normally means turning the slide away from the tailstock.

Ben. You do know this 27 deg thing. If you twist the top / compound around to 90 deg so that both the handles cross and top / compound are facing each other. Its 27 degrees back from there.

Its not 27 degrees from where it normally sits. That would be 63 degrees. If you don't have each handle remotely looking at you, your angles are wrong.

Turn the top slide square. Handle pointing at your guts. Turn it back 27 deg from square.

Now I'm confused. I'll take a photo tomorrow. Maybe I'm doing it wrong.

Ben

[Pete F]

That set-up took all of five seconds, I guess if I had taken 10 seconds I could have achieve a standard that even you could not fault. So maye still easy enough for most people. It is of course more than accurate enough for the threading procedure as described, splitting of hairs is not a requirement in this particular case.

Phil

Phil you seem to be missing the point that I'm trying to make here. Some people state that the compound NEEDS to be precisely 27.5 or 29.5 degrees in order to cut a thread in this way. I can say, with absolute 100% confidence, that is complete rubbish, and is a figure that is an urban myth perpetuated by the internet. If you choose not to believe me then let me ask you, why do you set 29.5, why not 29.6? Why not 29.55? Where do you stop with splitting things? For that matter, why not 30.000 degrees? The latter would in fact give the most precise result if the aim of the exercise is to cut on one flank only (all above assuming 60 degree threads). Have you verified the accuracy of the divisions on your top slide? How can you be sure, having wasted "10 seconds" precisely setting "half degrees" on your top slide that it's actually at that point? See where I'm going with this?

The whole point is that there are no hairs to split here. A threading tool is ground or made as a form tool, thus can cut on one flank, or both flanks. That translates to mean that the tool can be fed in to the work either with an offset of 30 degrees (cutting on the leading edge only) or plunged straight down (cutting on both edges equally). Clearly if it's possible to cut at both those extremes, then it's possible to cut anywhere in between too!

Hopefully without confusing those who are singing along at home, by offsetting the top slide and feeding in that way what we're doing is cutting a taper that is the thread angle. So at 30 degrees the taper is exactly equal to the thread angle and the tool would cut only on the leading edge. Happy days and what we want right? Yes, BUT .... for the reasons I outlined above we can't set EXACTLY 30 degrees on our compounds. Some may like to kid themselves that they can, and if that's the case they should set 30 degrees and not 29.5, as the former figure is what's desired. What's the problem with 30 degrees then? Because if you accidentally set more than 30 degrees the "taper" the tool is cutting will be incorrect and the form of the thread affected. Many people have done this at some stage by accidentally setting 60 degrees on their compound and then they say "What's wrong with my thread, it looks funny". So setting 30 degrees isn't a good ideas as you want a bit of wriggle room. So we normally set "just a tad" under 30 degrees in which case the leading edge of the work is still doing the majority of the cutting, while the trailing edge will just be skimming a bit of material off to match its form. Unfortunately in this digital age that "just a tad" has morphed into 0.5 degrees, a figure that's often spoke of as if handed down to Moses himself on a stone tablet! The whole point is IT DOESN'T MATTER what you set, just make sure it's less than half the thread angle.

Soooo why offset at all then, why not plunge straight in? Well the good news is you can. However there is less cutting force on the tool if it's cutting on one edge only. For those with big lathes or cutting only relatively fine threads it's no big deal, and just plunge it. However for those of us running glorified noodles, once the thread pitch starts getting larger our lathes will struggle so we need to give them all the help they can get. Personally I also find the whole process of resetting the cross slide to zero with each pass much easier to manage, but maybe that's just me and what I'm used to. I personally think it makes it much faster process with little chance of screw ups, if you pardon the pun, but to each their own.

The reason for going through all this is firstly I don't like to see rubbish perpetuated via the internet. Just because it's written down doesn't make it true, and I'd urge anyone who doesn't believe what I've said to try it for themselves. The other reason is that a good knowledge of what you're trying to achieve may have helped the OP get out of trouble. If there is a tailstock in the way with support, that's fine, then swing your compound around to "see more of your gut" and square up your tool to the work. You may not have had room for the top slide set at "29.5" degrees, that's fine, then swing it around until it fits and you can feed in. I'm with Phil, as I'm not sure why it wouldn't fit at around 30 degrees, but I wasn't there and sometimes these things happen. It may mean you can't take as large cuts, but that thread doesn't look too arduous, so a sharp tool would knock it off fine or it may mean less DOC and an extra pass. No biggie and no dies need be harmed in the process of making the thread

Hopefully that helps explain things a little better and will provide ammunition in order to get around difficulties when they pop up.

[morrisman]

For thread cutting info, the Workshop Practice book #3 'Screwcutting in the Lathe' by Martin Cleeve , is hard to beat . He covers just about every situation with easy to understand practical information . Mike

[DSEL74]

Just on a slight tangent… What was used to cut the thread, Ground HSS, a Carbide tip, or one of those holders with a spiral kind of thread cutting wheel?

I haven't advanced to thread cutting yet so can't really comment. But wouldn't the size of the lathe and type of tool post/holder have an effect on how close one can get to the tailstock?
I.e. If he had a big ass Dickson quick change with a short piece of HSS, and a fat centre combined all these things work against him?

Not debating anyones expertise, just trying to learn.

[pipeclay]

In answer to your question about size then yes it can/will play a part.

You can get extended nose live centres that will allow a tool to get much closer to the centre of the job.

You may find that the size of the tool post may play a part if it is excessively big, although people say you can do small work on a big lathe it is all relative to the size of the lathe.

The tool being used also can play a part, it could be to bulky/big to get in, you always have the choice to weld a piece of HSS to a bar and extend that out to give you more room.

I think in the case in question the use of an extended nose centre with a small bearing body may of worked, it may also of needed the HSS extension or even the use of a lantern tool holder if available.

You can even get some cemented carbide tipped tools that are quite thin for this purpose as well as tools that are off set.

[bwal74]

Yep, I had the compound around the wrong way. I don't know why I continually do that.

I was also using a carbide threading insert.

Ben.